Naga summed it up quite well. The professions in Ambitions take quite a bit of concentration and aren't suited to a large family. They are great fun, though, when you want to play a single Sim or a couple.
I think maybe the ability to register as self-employed was added to the base game through the update that was released with Ambitions. Ripster, you can probably confirm this for us. Are you patched up to the most recent update and can your Sims register to be self-employed as a painter, gardener, writer, etc.?